A printed circuit, also known by the acronym PCB (Printed Circuit Board), is a support that makes it possible to electrically link a set of electrical components. Such a printed circuit generally takes the form of a laminated plate. This printed circuit can be single-layer or multi-layer. A single-layer printed circuit comprises only a single metallization layer in which are printed conductive tracks that electrically connect the different electrical components together. A multilayer printed circuit comprises, on the other hand, a plurality of metallization layers. Such a printed circuit has at least two layers and, preferably, more than four or six layers. Hereinafter in this description, these multilayer printed circuits will be the focus of interest.
A metallization layer is one of the layers of the laminated plate forming the printed circuit in which are produced one or more conductive tracks that electrically connect the different electrical components together. This layer is planar and extends parallel to a plane of the laminated plate. Generally, the metallization layer is obtained by depositing a uniform layer of a conductive material, typically a metal such as copper, then etching this uniform layer to leave only the conductive tracks remaining.
The different metallization layers of the printed circuit are spaced apart mechanically from one another by insulating layers made of electrically insulating material. This insulating material exhibits a high dielectric strength, typically greater than 3 MV/m and, preferably, greater than 10 MV/m. For example, the electrically insulating material is produced from epoxy resin and/or glass fiber. The insulating layer generally takes the form of a rigid plate produced in a material that does not become viscous during its assembly with other layers. For example, the insulating layer can be produced from a thermosetting resin that has already undergone an irreversible thermosetting process.
The different layers of the multilayer printed circuit are assembled together, with no degree of freedom, using adhesive layers called “prepreg.”
A prepreg typically consists of a thermosetting resin impregnating a reinforcement, such as a fabric. Typically, the resin is an epoxy resin. During the fabrication of the printed circuit, the transformation of the thermosetting resin involves an irreversible polymerization that transforms the prepreg into a solid and rigid material that irreversibly bonds together the different layers of the printed circuit. Typically, each transformation occurs when the prepreg is heated to a high temperature and is compressed with a high pressure. Here, a high temperature is a temperature greater than 100° C. and, preferably, greater than 150° C. A high pressure is a pressure greater than 0.3 MPa and, typically, greater than 1 MPa.
The conductive tracks of the different metallization layers can be electrically connected via conductive bump contacts passing through the insulating layers. The conductive bump contacts are better known as “vias.” The vias generally extend at right angles to the plane of the layers. There are different ways of fabricating these vias. One of the most common ways is to produce a hole in the insulating layer or layers to pass through and then to cover the inner wall of these holes with a metal. These are called metalized holes.
A via does not necessarily pass through all the layers of the printed circuit. Thus, there are blind vias that emerge on a single outer face of the printed circuit. It is also possible to produce “buried” vias for example, using known technologies such as HDI (High Density of Integration). A buried via does not emerge on any of the outer faces of the printed circuit. For example, a buried via makes it possible to electrically connect conductive tracks produced in metallization layers embedded inside the printed circuit.
To obtain magnetic field or current sensors, it has already been proposed to fabricate them from a printed circuit notably comprising a magnetic core. A known magnetic field sensor produced from a printed circuit is described in Kubik, et al., “Magnetometer with pulse-excited miniature fluxgate sensor”, Journal of electrical engineering, vol. 57, No. 8/S, 2006, 80-83.
Such a magnetic field sensor is particularly accurate. On the other hand, this magnetic field sensor is very sensitive to the ambient conditions and, in particular, to temperature and mechanical stresses. For example, its accuracy varies as a function of the temperature, which degrades its performance.